The present invention relates to an automatic gap control or adjusting apparatus accompanying an excess adjusting prevention mechanism in a disc brake accompanying a parking brake device.
A conventional automatic gap control or adjusting apparatus accompanying an excess adjusting prevention mechanism of this type is disclosed, for example, in U.S. Pat. No. 3,633,712 and British Patent No. 1434369. The former example, that is U.S. Pat. No. 3,633,712, will be described with reference to FIG. 1. Pistons 1 and 2 disposed opposite to each other are slidably mounted in an opening 4 of a main body member 3. In order to operate the brake, pressurised fluid is introduced between these pistons 1 and 2 to move them away from each other, an end face of the piston 1 actuates a directly operated pad 5. The piston 2 actuates an indirectly operated pad (not shown) through a yoke 6. A gap control mechanism is constituted by a rod 9 and a supplemental piston 8 slidably provided in an opening 7 of the piston 1, being threadably connected to each other by a reversible screw structure thereof. A sealing ring 10 is provided on the opening 7 of the piston 1 and to seal a gap between the supplemental piston 8 and the piston 1. A small bleed opening 11 is disposed at the left end wall of the piston 1. A normal braking gap is given by a backlash formed along the longitudinal direction of the screw 14. When the pad 5 is worn out, a gap is produced, corresponding to the worn out value, between a conical surface 12 of the piston 1 and another conical surface 13 of the supplemental piston 8. In this condition, if the pressurised fluid is introduced between the pistons 1 and 2 to perform the brake, the supplemental piston 8 first moves leftward by a distance of the backlash of the screw 14, while the supplemental piston 8 does not rotate. If the supplemental piston 8 further moves leftward by the fluid, the supplemental piston 8 rotates owing to an operation of the reversible screw 14, whereas the rod 9 is prevented from rotating by a ring 15. The conical surface 13 of the supplemental piston 8 abuts against the conical surface 12 of the piston 1. The supplemental piston 8 continues rotating until the friction force between both the conical surfaces 12 and 13 increases to prevent the supplemental piston 8 from rotating. The supplemental piston 8 moves leftwardly by an advanced distance of the screw 14 corresponding to the rotation value of the supplemental piston 8, thereby increasing the length by means of the length control mechanism and as a result the gap produced by the worn-out of the pad is adjusted. If a temporary increase in the braking gap is produced due to some bending of the members of the brake mechanism during a strong or sudden braking operation, the length control mechanism would not be operated. Namely, when such strong braking is applied to the mechanism, the supplemental piston 8, the rod 9, and the piston 1 are moved together to compress the spring 16. When the braking operation is released, hydraulic pressure applied to the piston 1 and the supplemental piston 8 is decreased, so that the piston 1, the supplemental piston 8, and the rod 9 are integrally returned rightwardly by elastic force of the spring 16 and, therefore, the backlash of the screw 14 forms a suitable braking gap. Since the supplemental piston 8 does not rotate by the reversible screw 14 while the brake operation is released, the increased length of the length control mechanism described above is left as it is, so that any unsuitable or excessive gap due to the worn-out of pad of the brake mechanism does not exist. As a result, the length control mechanism does not react to the temporary gap.
Next, the latter conventional mechanism, that is GB No. 1434369 will be described with reference to FIG. 2. This is a hydraulic operative type disc brake which is provided with a braking cylinder 21 disposed at a part of the brake main body 20 and a cup-shaped piston 22 which is slidably provided in the cylinder 21. Pressurised fluid is introduced into the cylinder 21 through an inlet 23 in order to actuate a brake operation and the piston 22 moves through an opening end of the cylinder 21 whereas a stem 24 is kept still. The sleeve 25 moves, together with the cup-shaped piston 22 until a backlash is taken up, which is between the screw portion of the sleeve 25 of the length control mechanism and the screw portion 26 of the stem 24, which are screw-threaded to each other. When the brake pad 30 is worn out and the brake operation is actuated, it is necessary to further move the piston 22. In this case, a circular truncated conical outer surface 27 of the sleeve 25 constituting a circular truncated conical clutch surface and the circular truncated conical inner surface 28 of the piston 22 are separated when hydraulic pressure is applied therebetween. In this condition, the sleeve 25 rotates, following the movement of the piston 22, around the stem 24 due to the axial elastic force of the compression coil spring 29. As a result, the length control mechanism increases the combined length of the stem 24 and the sleeve 25. When excessive fluid pressure is applied to the cylinder 21 of the brake mechanism for an emergent stop of the vehicle, the fluid pressure applied to the actuating piston 34 which is urged so as to slide outwardly of an opening having a step by an elastic force of a coil spring 33 through a large diameter portion 32 of the stem 24 exceeds the elastic force of the coil spring applied to the piston 34, thereby moving the actuating piston 34 towards to the interior side of the large diameter portion 32 of the stepped opening formed in the stem 24. Consequently, a cooperative action of the actuating piston 34 and a ramp surface 36 of a peg 35 pushes the latter outwardly of a radial direction through a hole 37 communicating with the large diameter portion 32 of the stem 24. Therefore, the peg 35 abuts against an interior of the thread of the sleeve 25 so as to lock the sleeve 25, thereby preventing the sleeve 25 from rotating relative to the stem 24. The length control mechanism doesn't operate and the piston 22 disadvantageously move apart from the sleeve 25 threadably connected to the stem 24. Thus, when the fluid pressure in the cylinder 21 exceeds the predetermined pressure as described above, the combined length of the stem 24 and the sleeve 25 cannot be automatically adjusted.
The above brake mechanism further includes a disc rotor 31 and a central opening 38 formed on a closed end wall of the brake main body 20, through which the stem 24 passes.
As described above in detail, according to the former automatic gap control apparatus of the hydraulic type brake actuator, that is U.S. Pat. No. 3,633,712, through hole 11 is disposed on the left end wall of the piston 1 having the opening 7, the circular truncated conical surface 12 is provided at the peripheral edge of the inlet of the opening 7 at the right end of the piston 1, the circular truncated conical surface 13 engaged with the circular truncated conical surface 12 is formed at the right end portion of the supplemental piston 8, the piston 8 is sealingly and slidably fitted into the opening, the length control mechanism is constituted by combining the supplemental piston 8 and the rod 9 which are engaged with each other by a reversible screw 14 having a suitable backlash in order to eliminate the gap due to the pad worn-out. As a result, the automatic gap control apparatus is adapted not to react to any temporary increase of the brake gap caused by the bending of some construction members of the brake mechanism as if an emergent heavy braking is actuated. In order to obtain the function above, it would be required to precisely finish the inner surface of the opening 7 of the piston 1 and also the outer surface of the supplemental piston 8, as well as to assemble finely the screw connecting mechanism of the supplemental piston 8 and the rod 9 so as to have a suitable backlash. It has been troublesome and expensive to manufacture these parts. According to the latter automatic gap control apparatus, that is GB No. 1434369, it has a vehicle brake operating cylinder which comprises the brake main body 20 having the cylinder 21 an end of which opens, the cup-shaped piston 22 slidably mounted in the cylinder 21 and moves through the open end of the cylinder 21 by means of an operation of a fluid pressure, and the gap eliminating mechanism for automatically adjusting the gap due to the worn-out of the brake pad 30 by means of the thread connection of the sleeve 25 abutting against a bottom of the cup-shaped piston 22 and the stem 24 extending into the cylinder 21 through the central opening 38 formed in the closed end wall of the brake main body 20 As if an excessive fluid pressure is introduced into the cylinder 21 of the brake in order to suddenly stop the vehicle and a temporary brake gap increases due to the bending of the brake constructing member, the temporary brake gap is absorbed with the following operation. That is, the stem 24 has the actuating piston 34 installed in the large diameter portion 32 of the stepped hole and urged to slide out of the stepped hole by means of the coil spring 33. When an excessive fluid pressure is applied to the actuating piston 34 against the elastic force of the coil spring 33 and the actuating piston 34 moves inwardly, the peg 35 movably mounted in the radially directed hole 37 of the stem 24 locks the stem 24 and the threaded sleeve 25 to prevent them from relatively rotating and prevent, therefore, the length of the length control mechanism from increasing. Such length control mechanism has a complicated structure and troublesome to manufacture, resulting in the expensive products.